The conversion should take place before filtering, so it has to be in the texturing hardware. There is no need for sRGB specific sampling opcodes because sRGB is considered a property of the texture, not of the way you sample it.
See this, for example:
http://www.x.org/docs/AMD/R5xx_Acceleration_v1.5.pdf
Bit 21 of TXFORMAT0:
GAMMA 21 none Optionally remove gamma from texture before passing to
shader. Only apply to 8bit or less components.
POSSIBLE VALUES:
00 - Disable gamma removal
01 - Enable gamma removal